The subject matter described herein relates to an MRI apparatus and an RF pulse generating circuits. More specifically, the subject matter described herein relates to an MRI apparatus that can reduce noise introduced when generating RF pulses and an RF pulse generating circuit used to generate RF pulses in the MRI apparatus.
A high precision is required for RF pulse frequency. Therefore, a direct digital synthesizer (hereinafter referred to as a DDS) is used to generate RF pulses in a digital signal form (hereinafter referred to as digital RF pulse signals) and these pulses are converted into RF pulses in an analog signal form (hereinafter referred to as analog RF pulse signals) through a digital-analog converter (hereinafter referred to as a D/A converter). Hereinafter, “RF pulses” mean those that are transmitted from an RF coil.
When RF pulses are transmitted from an RF coil to which analog RF pulse signals have been applied, magnetic resonance signals are generated from a subject positioned in a magnetic field. The magnetic resonance signals are received by the RF coil and converted into magnetic resonance signals in a digital form (hereinafter referred to as digital magnetic resonance signals) through an analog-digital converter (hereinafter referred to as an A/D converter). Hereinafter, the magnetic resonance signals that have been received by the RF coil, but have not yet been converted into digital magnetic resonance signals by the A/D converter are referred to as analog magnetic resonance signals. “Magnetic resonance signals” plainly mentioned herein mean those generated from the subject and received by the RF coil.
When noise is superimposed on analog magnetic resonance signals, the noise appears as an artifact in an image reconstructed by the MRI apparatus and degrades the image quality. However, the receiving sensitivity of the RF coil must be as high as possible in order to receive faint magnetic resonance signals. This results in that noise is easily superimposed on analog magnetic resonance signals. Hence, to reduce noise impact, various means are taken. For example, a method for preventing image quality degradation resulting from superimposing of noise induced by a clock signal in a digital circuit on analog magnetic resonance signals is proposed (see, for example, Japanese Unexamined Patent Publication No. Hei 5(1993)-7570).
However, if it is possible to prevent noise that may be superimposed on analog magnetic resonance signals, this is most desirable.
Since RF pulses are transmitted to excite magnetic resonance signals, the frequency of the RF pulses is equal to the frequency of the magnetic resonance signals. Consequently, noise that is superimposed on analog magnetic resonance signals may be generated from a wiring path through which analog RF pulse signals are sent to the RF coil. To prevent this, measures for preventing noise generation, such as impedance matching, are taken for the wiring path for sending analog RF pulse signals after being converted by the D/A converter.
However, weak noise that is superimposed on analog magnetic resonance signals is also generated from a digital bus through which digital RF pulse signals are sent to the D/A converter.